For example, JP2009-71956A, corresponding to US2009/0066402A1, describes a gate drive circuit for driving a power switching element, such as an insulated gate bipolar transistor (IGBT), as a load. The described gate drive circuit has a first turn-on power supply circuit for turning on a power switching element. The first turn-on power supply circuit has a first turn-on power source as a power source dedicated for the first turn-on power supply circuit.
The first turn-on power supply circuit supplies a voltage equal to or lower than a predetermined level to a gate of the power switching element to turn on the power switching element. Therefore, an adverse influence, such as a characteristic change, to the power switching element is reduced.
The above gate drive circuit is an example, and some other types of the gate drive circuit, that is, a driver circuit are widely known.
In a switching element driven by such a driver circuit, a time to turn on the gate depends on a capacity of the driver circuit. The shorter the rising time is, the more the speed of rising the gate increases.
For example, assuming that the capacity of the gate is C, the value of the constant current to charge the gate is I, and an on-voltage where the gate becomes an on state is V, the rising time T for turning on the gate is expressed as T=(C×V)/I. The constant current is supplied to the driver circuit from an external device.
According to the above expression, the rising time T is shortened by increasing the value I of the electric current supplied to the driver circuit or by reducing the gate capacity C or the on-voltage V. That is, by shortening the rising time T, the rising speed of the gate increases.
The gate capacity C and the on-voltage V are uniquely decided depending on the size or the manufacturing process of the switching element. Therefore, the rising time T is adjusted by the value I of the constant current.
It is considered to shorten the rising time T by increasing the value I of the constant current. However, the increase in the value I of the constant current results in an increase in current consumption in the driver circuit. Particularly, if the constant current having the increased value I is continuously supplied to the driver circuit even after the gate reaches the on state, the consumption current in the driver circuit is increased.
For example, JP2009-11049A corresponding to US2009/0002054A1 describes a gate drive apparatus that drives a gate of a power element as a load, such as an IGBT or a MOSFET, with a constant current. In the gate drive apparatus, a constant-current-pulse gate drive circuit is connected to the gate of the power element.
When the constant-current-pulse gate drive circuit is operated in accordance with a control signal, the constant current is supplied to the gate of the power element from the constant-current-pulse gate drive circuit. Since the gate of the power element is supplied with an electric charge, a gate voltage rises, and thus the power element turns on.
In such a structure, if an overshoot occurs when the gate of the power element reaches a fully on state, the power element will be damaged. Therefore, it is considered to connect a clamp circuit, which clamps the gate voltage on a constant voltage, to the gate of the power element, thereby to restrict the overshoot of the power element and to protect the power element.
However, a current path through which the constant current passes is formed in the clamp circuit at a timing where the gate voltage reaches the voltage of the clamp circuit. Therefore, the constant current supplied to the power element will be continuously supplied also to the clamp circuit as long as the constant current is continuously supplied to the power element. As a result, the current consumption will be increased.
For example, JP2004-72424A describes a gate drive circuit of a MOS gate transistor which can shorten a turn-on time for a high power transistor and also can reduce internal power consumption. However, in JP2004-72424A, the value of a constant current is not changed.